1. Field of the Invention
This invention relates to a circuit arrangement for decoding a frequency-modulated stereo radio signal, in which a FM modulator is followed by a stereo decoder and a matrix in which, by sum and difference formation, two reproduction signals are formed in the matrix from a sum signal component of the output signal of the FM demodulator and from a difference signal component converted from an auxiliary carrier frequency state into the audio frequency state by a synchronous demodulator.
2. Description of the Prior Art
The so-called MPX signal supplied by the FM demodulator in a radio receiver contains three components. The first component is a sum signal (L+R) consisting of the left-hand item of information L and the right-hand item of information R in the audio-frequency range from 30 Hz to 15 kHz. The second component is a difference signal (L-R) consisting of the left-hand item of information L and the right-hand item of information R which is modulated onto a suppressed auxiliary carrier of, for example, 38 kHz. The frequency band for the difference signal (L-R) extends, with the lower and upper side bands, from, for example, 23 kHz to 53 kHz. A third component serves to transmit a pilot tone of 19 kHz, for example, which allows the 38 kHz auxiliary carrier to be regenerated in the stereo decoder.
The synchronous demodulator converts the difference signal (L-R) from the 38 kHz state into the audio-frequency state with the help of the 38 kHz auxiliary carrier. Both of the signals in the audio-frequency state--the sum signal (L+R) and the difference signal (L-R) are combined in the matrix to form the two reproduction signals U.sub.L and U.sub.R which corresponds to the left-hand information L and the right-hand information R, in accordance with the following equations: EQU (L+R) + (L-R) = 2L; and EQU (L+R) - (L-R) = 2R.
a prerequisite of the fulfillment of these equations for the left-hand information L and the right-hand information R, i.e. that the two reproduction signals U.sub.L and U.sub.R should contain exclusively the left-hand information L and the right-hand information R, respectively, and that there should be no crosstalk between these signals is that the sum signal (L+R) and the difference signal (L-R) should be identical in terms of amplitude and phase. As, however, the sum signal (L+R) is transmitted in the audio-frequency state, and the difference signal (L-R) is transmitted in the auxiliary carrier frequency state, a nonlinear frequency response manifests itself in the form of cross talk between the two reproduction signals U.sub.L and U.sub.R. If it is assumed that the FM demodulator possesses a linear frequency response, but that it is connected via a connection line to the stereo decoder, then at the input of the stereo decoder, the amplitudes of the higher frequencies are reduced in relation to those of the lower frequencies in that the connection lines forms a RC low pass filter. This means that the difference signal (L-R) is somewhat lower in terms of amplitude than the sum signal (L+R), and that the two signals no longer agree in terms of phase. Consequently, the two above-mentioned equations are no longer fulfilled, and an undesired cross talk occurs between the two reproduction signals U.sub.L and U.sub.R. In an unfavorable situation, nonlinearities of the FM demodulator will serve to further increase such an effect.